Penicillin-sensitive DD-carboxypeptidases from Streptomyces strains R39 and K11

The two penicillin-sensitive DD-carboxypeptidases from Streptomyces R39 and K11 are anionic at pH 8. They specifically recognize a C-terminal L-R3-D-alanyl-D sequence with a long side chain at the R3 position. The two enzymes differ from each other with respect to: (1) the effects of ionic strength on activity, (2) the influence exerted on activity by the presence of a free amino group at the end of the L-R3 side chain, (3) the K3 and Vmax values. Enzyme K11 has Km values which are high for both good and poor substrates. The enzyme efficiency reflects itself in Vmax values which are high for good substrates and low for poor substrates. Enzyme R39 has Km values which are low for good substrates. The enzyme efficiency toward various substrates reflects itself in the Km and, to a lesser extent, in the Vmax values, (4) the effects of penicillin. Kinetically, inhibition of enzyme K11 by penicillin is competitive. On the contrary, inhibition of enzyme R39 by penicillin is noncompetitive and increasing penicillin concentrations cause disproportionate decreases in the catalytic rate. Noncompetitiveness cannot be attributed to an irreversible inactivation of the enzyme by penicillin.